The purpose of this project is to define the pathogenesis of the hantavirus pulmonary syndrome (HPS) which we hypothesize is due to immunopathology triggered by hantavirus specific immune responses. The capillary leak syndrome occurring in the lungs of the infected patients, the increase in hematocrit and decrease in platelets are reminiscent of the capillary leak syndrome, hemoconcentration and thrombocytopenia which is observed in cases of dengue hemorrhagic fever (DHF). We reported earlier that individuals experiencing DHF have significantly higher levels of T cell activation than do individuals experiencing uncomplicated dengue fever. Levels of soluble CD8, soluble IL2 receptors and soluble CD4 are significantly higher in children with DHF than in children with uncomplicated dengue fever. These similarities in clinical and laboratory measurements in patients with the HPSD DHF syndromes led us to hypothesize that the HPS syndrome is caused by massive capillary leak syndrome in the lungs of infected individuals as a result of marked T cell activation in the infected target organ which is the lung. Unlike DHF which usually occurs in individuals who have been infected previously with the serologically related dengue virus, HPS appears to be the result of a primary hantavirus infection in a non-immune individual but we hypothesize that the underlying mechanisms of immunopathogenesis are similar. We have begun to define hantavirus specific T cell responses in the patients with HPS. We have succeeded in isolating T cell clones from patients with HPS and have defined a human CD8+T cell epitope on the FCV nucleocapsid protein which does not cross-react with other hantaviruses. In the proposed experiments we will define the in vivo activity of mononuclear cell infiltrates in lung tissues, pleural effusions and in peripheral blood cells from patients with HPS by identifying the phenotype of these cells, defining cytokine production and TCR usage by these cells and their relationship to virus infected lung endothelial cells. We have developed techniques for identifying specific cytokine producing cells on frozen tissues obtained from pathological specimens. We will determine whether blood and pleural fluid cells obtained from patients with HPS have detectable HV-specific cytotoxic T cell activity. We will measure the virus burden in the same samples of lung tissues, pleural effusion and peripheral blood by quantitative PCR. We will assess whether the outcome of infection correlates with results of studies of immune responses and/or with virus burden. The results of these studies should provide fundamental, novel information on the pathogenesis of HPS and serve as a basis for consideration of prevention of therapeutic measures.